Method of fabricating a coplanar waveguide device including removal of spurious microwave modes via flip-chip crossover

ABSTRACT

A coplanar waveguide device includes a coplanar waveguide structure disposed on a substrate, at least one qubit coupled to the coplanar waveguide structure and an add-on chip having a metallized trench, and disposed over the substrate.

DOMESTIC PRIORITY

This application is a divisional of U.S. patent application Ser. No.13/838,324, filed Mar. 15, 2013, the disclosure of which is incorporatedby reference herein in its entirety.

FEDERAL RESEARCH STATEMENT

This invention was made with Government support under Contract No.:W911NF-10-1-0324 awarded by the U.S. Army. The Government has certainrights in this invention.

BACKGROUND

The present invention relates to coplanar waveguides, and morespecifically, to universal flip-chip crossover systems and methods forremoval of spurious microwave modes in coplanar waveguides.

Coplanar waveguide (CPW) layouts are becoming commonplace insuperconducting qubit quantum computing designs. Superconducting CPWresonators can be made with high quality factors (Q), in excess of100,000 or more. Furthermore, the large ground planes help confine theelectromagnetic mode in a relevant region of interest.

A typical CPW resonator includes a center conductor of defined lengthand two adjacent ground planes. Simple single CPW resonator structuresinclude either one or two ports, as well as two large continuous groundplanes. The resonator is addressed via capacitive coupling to the portsvia CPW microwave feedlines. Depending on half-wave or quarter-waveconfiguration, determined by how the center conductor is terminated(i.e., open or short), the cavity and its relevant modes arepredictable. However, microwave electromagnetic (EM) simulations revealthe presence of other spurious on-chip modes, including differentialslotline modes, which can travel along the large ground planes. Formultiple CPW resonators on the same chip, more and more such parasiticEM modes are present since each CPW resonator contributes additionalground planes that are not well connected and therefore are notisopotential at different frequencies. For most applications, especiallythose involving superconducting circuits, the presence of such modes isdetrimental to the operation and measurement of the superconductingcircuits.

Conventional techniques for removing unwanted modes involve the use ofwire-bond straps, using an aluminum bond wire to attach from one pieceof ground plane onto the other, spanning across the center conductor.Often just a few well-placed bonds can greatly mitigate the presence ofthese modes. However, this is only a trivial solution for simple designssuch as those for a single CPW cavity. For more complicated structureswire bonds are not feasible, often due to space constraints andrepeatability concerns.

Another method to deal with parasitic modes uses microfabricatedshorting straps connecting the ground planes together, therebyperforming the same role as the previously mentioned wirebonds butavoiding the tedious manual wirebonding step. These straps arefabricated with additional photolithography steps. One of the drawbacksof this method is that a dielectric is required for the crossover inorder to prevent shorting to the center conductor. For superconductingcircuits at low temperature and low power, most dielectrics have arelatively large dielectric loss tangent and thus could reduce theresonator quality factor and/or qubit performance. In addition, it isdifficult to create such a crossover which is adequately spaced from thecenter conductor so that the resonant properties of the CPW are notaffected.

Other techniques exist as well but are even less feasible given thecurrent state-of-the-art. One possibility involves through-vias placedinto the substrate. The top ground plane is directly tied to a bottomground plane. Sufficiently closely spaced through-vias then help tie allground structures together. However, although the steps are well knownto make through-vias, the extra processing steps may be prohibitivelytime consuming and could also be impossible for substrates such assapphire, commonly used in superconducting circuits. Similarly, embeddedstripline designs involve a multi-layer process that could degrade thedevice performance.

SUMMARY

Exemplary embodiments include a coplanar waveguide device, including acoplanar waveguide structure disposed on a substrate, at least one qubitcoupled to the coplanar waveguide structure and an add-on chip having ametalized pattern-trenched surface, and disposed over the substrate.

Additional exemplary embodiments include a coplanar waveguide device,including a first chip including a patterned coplanar waveguide and asecond chip disposed on the first chip, the second chip having ametalized pattern-trenched surface that remove spurious microwave modesfrom the first chip.

Further exemplary embodiments include a method of fabricating a coplanarwaveguide device, the method including fabricating a qubit circuit chip,fabricating an add-on chip configured to remove spurious microwave modesfrom the qubit circuit chip and disposing the add-on chip on the qubitcircuit chip.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a cross-sectional view of an example of an exemplaryCPW device for removing spurious microwave modes;

FIG. 2 illustrates a cross-sectional view of another example of anexemplary CPW device for removing spurious microwave modes, including asignal access or control line for introducing and/or reading microwavesignals from the device below;

FIG. 3 illustrates a perspective view of another example of an exemplaryCPW device for removing spurious microwave modes;

FIG. 4 illustrates a top plan view of an exemplary CPW device forremoving spurious microwave modes;

FIG. 5 illustrates a flowchart of a method for fabricating a CPW devicecap structure in accordance with exemplary embodiments;

FIG. 6A illustrates a starting structure for an exemplary CPW device capstructure;

FIG. 6B is an intermediate structure for an exemplary CPW device capstructure;

FIG. 6C is an intermediate structure for an exemplary CPW device capstructure;

FIG. 6D is an intermediate structure for an exemplary CPW device capstructure; and

FIG. 6E is an intermediate structure for an exemplary CPW device capstructure.

DETAILED DESCRIPTION

In exemplary embodiments, the systems and methods described hereinimplement a cap structure and substrate that support qubits for quantumcomputing applications. The systems and methods described herein preventcapacitive coupling between remote sections of the circuit and removespurious microwave modes that are detrimental for quantum computingapplications. Qubits in quantum computing circuits include, but are notlimited to, thin film elements such as Josephson junctions,interdigitated capacitors and the like. These elements form ananharmonic oscillator and serve to store information. Capacitivecoupling between the qubit and CPW resonators allows for interrogationof the state of the qubit, and for actively modifying this state. It isalso important for two main reasons that the qubit be isolated from theneighboring elements. First, the qubit coherence time suffers ifinadvertent coupling exists between the qubit and other resonantsystems, limiting the amount of time available to perform gateoperations on the qubit. Second, the qubit is nominally identical toother qubits in the two-dimensional lattice forming the quantumcomputing system. In order to operate properly, only the resonators withintentional coupling can interact with the qubit, so that the timeevolution of the state of the qubit proceeds for the plannedmeasurements. As such, effective isolation of the qubits from thecircuit (discounting intentionally designed coupling) is necessary.

FIG. 1 illustrates a cross-sectional view of an example of an exemplaryCPW device 100 for removing spurious microwave modes. In exemplaryembodiments, a standard qubit circuit is disposed on a substrate 106,implementing fabrication techniques that yield superconducting qubitdevices with acceptable coherence time metrics. The exemplary CPW device100 includes two ground planes 102, 104 and a center conductor 103.However, it will be appreciated that the ground planes and centerconductor configuration described in this example is for illustrativepurposes and that other structures including a superconducting qubit andits associated planar structure could be implemented instead of thecenter conductor of a CPW. In exemplary embodiments, an add-on chip 101(i.e., cap) is placed on top of the standard chip and substrate 106. Theadd-on chip 101 includes a cavity 105 that has been etched into theadd-on chip 101. In exemplary embodiments, a continuous metallization108 provides good grounding between the ground planes 102, 104 via alarge capacitance that behaves like a short at microwave frequencies.

FIG. 2 illustrates a cross-sectional view of another example of anexemplary CPW device 200 for removing spurious microwave modes. Theexample in FIG. 2 also illustrates a standard qubit circuit that isdisposed on a substrate 206, implementing fabrication techniques thatyield superconducting qubit devices with acceptable coherence timemetrics. The exemplary CPW device 200 includes two ground planes 202,204 and a center conductor 203. In exemplary embodiments, an add-on chip201 (i.e., cap) is placed on top of the standard chip and substrate 206.The add-on chip 201 includes a CPW center-conductor matching trench 205that has been etched into the add-on chip 201. In exemplary embodiments,a continuous metallization 208 provides good grounding between theground planes 202, 204 via a large capacitance that behaves like a shortat microwave frequencies. The example in FIG. 2 further includes controlsignal line 207 that can provide high frequency microwave signals to thecenter conductor 203. It will be appreciated that the center conductor203 can be replaced with a qubit so that the control signal line 207supplies a qubit with a microwave control signal.

FIG. 3 illustrates a perspective view of another example of an exemplaryCPW device 300 for removing spurious microwave modes. The example inFIG. 3 is similar to the examples of FIGS. 1 and 2. The example in FIG.3 illustrates further details of a standard qubit circuit 310 that isdisposed on a substrate 306. The standard qubit circuit 310 includes aground plane 302, 304 disposed on the substrate 306. The standard qubitcircuit 310 further includes a portion of resonator 311 disposed on thesubstrate 306. The resonator 311 is connected to a drive port 312, whichis implemented to drive the resonator and nearby coupled qubits (notshown). The exemplary CPW device 300 further includes an add-on chip 301and cavity 305, similar to the add-on chips described herein. FIG. 3illustrates the add-on chip 301 covering only a portion of the substrate306 and standard qubit circuit 310. As further described herein, theadd-on chip 301 can also cover the portion of the resonator 311 anddrive pad 312. FIG. 3 further illustrates that the substrate 306 caninclude alignment marks 320, which allows the add-on chip 301 to bealigned with the substrate 306 during assembly of the CPW device 300.

FIG. 4 illustrates a top plan view of an exemplary CPW device 400 forremoving spurious microwave modes. FIG. 4 shows a standard qubit circuit410 disassembled from a corresponding add-on chip 401. FIG. 4illustrates further details of the standard qubit circuit 410 includingportions of a resonator 411, drive pads 412 and qubit pockets 413 intowhich qubits are disposed. FIG. 4 also illustrates further details ofthe add-on chip 401 showing that the cavity 405 includes severaldifferent portions that match the features (e.g., the resonator 411, andthe qubit pockets 413) of the standard qubit circuit 410. As describedherein, the standard qubit circuit 410 can be fabricated from a suitablemetal such as but not limited to niobium nitride (NbN), titanium nitride(TiN), niobium (Nb) and aluminum (Al), which is patterned on a substratesuch as silicon (Si). The metal is patterned to the corresponding CPWstructures (e.g., the resonator 411), the drive pads 412 and the qubitpockets 413. As also described herein, the add-on chip 401 includes acontinuous metallization 408 which can be Nb, Al or other materials.Further details of fabrication are described herein.

In exemplary embodiments, the systems and methods described hereintherefore provide a cap structure (i.e., the add-on chips) that followsthe structure of the standard qubit circuits on the respectivesubstrate. Capacitive coupling between active elements on the standardqubit circuit and the cap structure is controllable so that CPW deviceproperties do not fluctuate to an unacceptable amount due to variationsin cap dimension, which are expected or which occur in manufacturing. Asdescribed herein, the cap structure can cover portions of the standardqubit circuit or the entire standard qubit circuit.

In exemplary embodiments, the cap structure is sized to achieve variousfunctions. First, the cap structure provides an additional capacitanceto the qubit LC oscillator. In design, the capacitance of the qubit isreduced in order that the net capacitance (and hence frequency) remainsconstant. However, since this additional capacitance is determined byproximity of the cavity within the cap structure to the qubit structure,and any errors in size or placement of the cap contributes to afrequency spread. Since qubits are designed to operate at precisefrequency, the cap structure is fabricated large enough so that the capwalls are far enough away so that physical placement or size errorscontribute relatively less to the qubit capacitance. As such, there isalso a reduced size that the cap structure can be. As such, the cap isfabricated deep enough so that the distance from the qubit to the cavitywalls either laterally or perpendicular to the surface are about thesame. Similar considerations are made for the CPW since frequencieswould shift with the cap being present as well. As a further refinement,the capacitive load from the cap along the extended structures such as acoplanar wave guide resonator should be design so that the capacitanceper length is approximately constant along the length, so that the modeshape is not significantly altered. Second, the qubit couples to theenvironment via the capacitance to the ground shield provided by the capstructure. Any source of dissipation in the cap structure or connectedstructures causes decoherence in the qubit. Decoherence is undesirablein qubit operation. As such, reducing the capacitance to the point wherethis decoherence (due to Purcell loss) drives the cap structure tolarger dimensions.

In exemplary embodiments, the cross section of the exemplary capstructures described herein can be any suitable shape such as, but notlimited to, rectangular, depending in part on the fabricationtechniques. For example, reactive ion etching (RIE) results in cleanprofiles with substantial, uniform depth. Coating the cap structure withsuperconducting metal is performed if the cap structure is made with Si.For example, the cap structure could be completed by use ofhigh-pressure sputter deposition process to ensure coating on thesidewalls.

In exemplary embodiments, the exemplary cap structures can thus befabricated without requiring fabrication intensive techniques or novelmaterials. As described herein, a standard qubit circuit with CPWlayouts and other qubit designs patterned in Nb and Al on a SiO₂/Sisubstrate can be implemented with the exemplary cap structures describedherein. In exemplary embodiments, the add-on chip is placed upside downon top of the standard qubit circuit chip. The add-on chip can beanother piece of SiO₂/Si substrate, which is then etched in thelocations corresponding to the CPW center striplines, launch pads, andactive elements (e.g., qubits) in the standard qubit circuit chip.Subsequent to etching the cavities in the cap structure, the entireSiO₂/Si surface is deposited with blanket metallization (e.g. Nb). Withsufficient coverage, a continuous side-wall coating into the trench ismade. The electrical connection to the circuit chip is possible at DCcurrent because of the native dielectric on the metal surfaces. Even ifa DC ground connection is not made, for high frequency currents theeffective capacitance formed between the metal of the add-on chip andthe ground of the standard qubit circuit chip tie all ground planestogether. No explicit dielectrics are (but can be) necessarilyimplemented and the circuit chip can be fabricated using the standardtechniques yielding high performing devices.

In exemplary embodiments, as described herein, the add-on chip can beimplemented to bring multiple control signals to the entire CPW device.Losses in the add-on chip above the Nb ground plane do not adverselyimpact device performance and as such a more complicatedphotolithographic process is possible for the add-on chip, opening thedoor for scalable chip design for quantum computing. In addition, itwill be possible to stack quantum computing chips and use the cap as adouble-sided spacer which serves to both isolate the chips and toprovide a path for signal wires to connect neighboring chips. It will beappreciated that various fabrication techniques exist which could beused to fabricate the additional control signals that could be used toapply quantum computing operations on qubits. An example of afabrication technique is now described in further detail.

FIG. 5 illustrates a flowchart of a method 500 for fabricating a CPWdevice cap structure in accordance with exemplary embodiments. At block505, a hardmask is fabricated. For example, the hardmask can be SiO₂/Siwith patterns for etching at locations corresponding to all CPW centerconductor and gap channels, all qubit circuit pockets, and any othernon-ground plane elements such as coupling capacitors on the standardqubit circuit. FIG. 6A illustrates a starting structure 600 for anexemplary CPW device cap structure. At block 510 of FIG. 5, thesubstrate 606 for the cap structure is patterned. For example, thesubstrate can be a 660 μm thick Si wafer. FIG. 6B is an intermediatestructure 650 for an exemplary CPW device cap structure. A layer 651 ofSiO₂ is grown on the substrate 606 as a hard mask. For example, the SiO₂layer can be 1 um of SiO₂. FIG. 6C is an intermediate structure 655 foran exemplary CPW device cap structure. At block 515 of FIG. 5, thepatterns (represented by pattern 656) from the etch mask of block 505are formed on the SiO₂ layer 651 and substrate 601. Standardphotolithography and etching techniques can be implemented to transferthe patterns onto the SiO₂ layer. For example, a resist scheme can be600 Å of developable bottom anti-reflective coating (DBARC) followed by0.45 μm UV110 positive photoresist. In addition, the patterns aretransferred via reactive ion etching (RIE). Photoresist can be strippedusing an appropriate solvent. In addition, tetramethyl-ammoniumhydroxide (TMAH) can be used for deep trench etching (e.g., 5 μm to 100μm depth). FIG. 6D is an intermediate structure 660 for an exemplary CPWdevice cap structure. At block 520 of FIG. 5, the layer 651 (i.e.,hardmask) is removed from the substrate 606. For example, a wet etch(i.e., buffered oxide etch typically containing hydrofluoric acid) canbe implemented to remove the SiO₂.

FIG. 6E is an intermediate structure 665 for an exemplary CPW device capstructure. At block 525 of FIG. 5, the substrate 606 is metallized witha metallic layer 667, such as but not limited to Nb, as describedherein. In exemplary embodiments, fabrication techniques are implementedin order to maintain connectedness throughout the deposition so that allareas, including sidewalls, of the substrate are coated. For example,the metallic layer 667 can be blanket sputtered in a uniform layer ofabout 200 nm to 500 nm. At block 530 of FIG. 5, other fabrication stepsare performed. It can be appreciated that on a single wafer, multipleadd-on chips can be fabricated. As such, add-on chips are individuallydiced from the wafer, the actual size of the add-on chips being sizedthe same or smaller than the standard qubit circuit chips as describedherein depending on which components of the standard qubit circuits areto be covered by the add-on chips. Regular circuit chip fabrication isperformed as in current standard processes, with the addition ofalignment marks on the optical lithography mask to indicate the locationof placement for the add-on chips. Assembly involves micromanipulationusing probes to accurately place roof chips and affix using amounts ofpoly(methyl methacrylate) (PMMA) or cyanoacrylate at the corners oralong the edges of the roof chip to circuit chip interface. In otherexemplary embodiments, possible assembly techniques also include etchingpre-patterned holes in both the standard qubit circuit and add-on chips,and performing alignment via a micro pin placed through both chips.Although other Al wirebonds can be implemented, the presence of theadd-on chip and standard qubit circuit chip often allows no further needof Al wirebonds.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. A method of fabricating a coplanar waveguide(CPW) device, the method comprising: fabricating a qubit circuit chip;wherein fabricating the qubit circuit chip comprises: patterning a CPWstructure; fabricating at least one qubit; and coupling the at least onequbit to the CPW structure; fabricating an add-on chip configured toremove spurious microwave modes from the qubit circuit chip; whereinfabricating the add-on chip comprises: defining a trench in a substrate,the trench matching an outline of the at least one qubit, wherein thetrench comprises an opening; and metalizing the trench and thesubstrate; disposing the add-on chip on the qubit circuit chip such thatthe opening in the trench faces the qubit circuit chip; and wherein thepatterning of the CPW structure comprises forming ground planes on thequbit circuit chip and wherein the ground planes are exposed by theopening.
 2. The method of claim 1, wherein the trench and substrate aremetallized with niobium (Nb).
 3. The method of claim 1, whereinpatterning the CPW structure further comprises: forming a resonator;forming a drive pad coupled to the resonator; and forming a qubit pocketcoupled to the resonator.